Method for producing alloy steel

ABSTRACT

An improved method for producing alloy steel, comprising the steps of: charging a furnace with materials comprised of iron, carbon and sulfur, and at least one metal from the group consisting of cromium and nickel; melting the charge; desulfurizing the melt to a sulfur level below 0.04 percent, generally below 0.03 percent; decarburizing the desulfurized melt to a carbon level below 0.15 percent, generally below 0.07 percent; deoxidizing the melt; and casting the melt. In addition the method often includes the additional step of finally adjusting the alloy composition for certain elements, such as chromium, prior to decarburizing.

United States Patent [191 Ramachandran et al.

[ June 11, 1974 METHOD FOR PRODUCING ALLOY STEEL [73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

22 Filed: Sept. 29, 1970 211 App]. No.: 76,592

[52] US. Cl 75/12, 75/49, 75/1305 [51] Int. Cl. C2l 5/52, C2lc 7/10 [58] Field of Search 75/49, 130.5, 12

[56] References Cited UNITED STATES PATENTS 2,041,333 5/1936 Gruber et al 75/49 UX 2,068,785 l/l937 Bain et a1 75/49 X 2,071,942 2/1937 Rohn 75/49 X 2,093,666 9/1937 Vogt 75/49 X 2,110,067 3/1938 Heuer 75/49 X 2,665,982 1/1954 Crego 75/51 2,993,780 7/1961 Allard 75/49 3,003,865 10/1961 Bridges 75/l30.5 X 3,172,758 3/1965 Jandras 75/l30.5

3,198,624 8/1965 Bell et a1 75/130.5 3,207,596 9/1965 Pinches 75/60 3,218,157 11/1965 Dobbowsky et al. 75/l30.5 X 3,336,132 8/1967 McCoy 75/49 3,420,657 l/l969 Hansen.. 75/l30.5 3,459,867 8/1969 Estes 13/9 3,615,348 10/1971 Tanczyn 75/49 X Primary Examiner-L. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [57] ABSTRACT An improved method for producing alloy steel, comprising the steps of: charging a furnace with materials comprised of iron, carbon and sulfur, and at least one metal from the group consisting of cromium and nickel; melting the charge; desulfurizing the melt to a sulfur level below 0.04 percent, generally below 0.03 percent; decarburizing the desulfurized melt to a carbon level below 0.15 percent, generally below 0.07 percent; deoxidizing the melt; and casting the melt. In addition the method often includes the additional step of finally adjusting the alloy composition for certain elements, such as chromium, prior to decarburizing.

20 Claims, No Drawings METHOD FOR PRODUCING ALLOY STEEL The present invention relates to a method for producing alloy steel and involves a sequence of steps which improves efficiency and reduced costs. The method centers around a vacuum decarburizing treatment and a sequence of steps which places desulfurizing ahead of decarburizing. It additionally pertains to a process in which a final adjustment of alloy composition is made prior to decarburizing for most elements; e.g., chromium, nickel, molybdenum, copper and iron, with the exception of those highly reactive elements; e.g., titanium and aluminum.

It is advantageous to desulfurize prior to decarburizing as desulfurizing is more effective when the oxygen level of the melt and slag are low and since lower oxygen contents are present when the carbon and silicon contents are higher, as they are prior to decarburizing. ln addition, early desulfurizing; i.e., desulfurizing prior to decarburizing, permits the use of carbides in the slag; e.g., calcium carbide and silicon carbide, as there is little worry about the consequences of carbon pickup early in the process. Carbides are advantageous in the slag since they act to return reducible oxides such as those of manganese, chromium, vanadium, tungsten, iron, etc., from the slag to the metal. Furthermore, early desulfurizing eliminates the need for a slag later in the process and shortens the period of time during which slag is present. Slags adversely affect and destroy refractory linings.

A final adjustment of alloy composition prior to or during decarburizing is advantageous as it permits adjustment additions to be made in the form of high carbon ferro-alloys whereas additions made subsequent to decarburizing generally have to be in the form of low carbon ferro-alloys. High carbon ferro-alloys, which are considerably less expensive than low carbon ferroalloys, can be added prior to or during decarburizing as there is little worry about the consequences of carbon pickup at those stages in the process. Adjustment additions are necessitated by inaccuracies which exist in the analysis of scrap which is charged into the furnace, by melt-down losses which cannot be easily predicted and by the fact that compensation is generally necessary to account for some small degree of metallic oxidation which occurs during vacuum decarburizing.

It is accordingly an object of this invention to provide an improved method for producing corrosion resistant steel. Background information pertaining to prior art methods can be obtained from U.S. Pat. No. 3,336,132 which issued on Aug. 15, 1967.

In its broadest aspects the method of the present invention comprises the steps of; charging a furnace with materials comprised of iron, carbon and sulfur, and at least one metal from the group consisting of chromium and nickel; melting the charge; desulfurizing the melt to a sulfur level below 0.04 percent, generally below 0.03 percent; decarburizing the desulfurized melt to a carbon level below 0.15 percent, generally below 0.07 percent; deoxidizing the melt and casting the melt.

The charge contains materials comprised of iron, carbon and sulfur, and at least one metal from the group consisting of chromium and nickel. It can also contain other elements such as molybdenum, cobalt, copper, and tungsten which might be desired in the steel being produced, as well as slag forming ingredients; e.g., lime, that go into the formation of a melt-down slag which aids in desulfurizing. In general, the charge contains at least 90 percent of the heat weight; i.e., the weight of the metal ready to be cast. A substantial portion of the remainder of the heat weight; i.e., the remaining 0 10 percent comes from cool-off additions; e.g., additions to lower the melt temperature into the tapping range. These figures compare quite favorably with prior art processes in which substantially smaller percentages; e.g., percent, of the heat weight were charged. Prior art processes required larger amounts of cool-off and final additions as well as reduction mixes. Reduction mixes were added after decarburizing to effect the return of a portion of the metallic values, from the slag to the melt, which oxidized during decarburizing. They contained reducing material; e.g., ferrosilicon and/or ferrochromium silicon, and cool-off additions (the cool-off additions had to be the expensive, high quality, low carbon type since they were added after decarburizing).

Any of the well known means can be used to melt the charge. Illustrative means are electric arc furnaces and induction furnaces. Slag forming ingredients are generally part of the charge when an electric arc furnace is employed and are generally not part of the charge when an induction furnace is employed. An electric arc furnace is designed to accommodate slag whereas an induction furnace is not. The electric arc furnace has a larger circumference than the induction furnace and therefore subjects less refractory lining to the slags which adversely affect and destroy them. At times it is desirable to use electric power to partially melt the charge and then to use an oxygen containing gas to finish melting. This procedure can lower the silicon content of the melt to a level at which only a small degree of silicon oxidation occurs during the subsequent decarburizing treatment of this invention. This in turn decreases the amount of slag which forms during decarburizing and therefore reduces the amount of refractory lining deterioration. Oxygen should, however, only be used if it does not lead to excessive metallic losses of iron and chromium. The carbon content of the melt is generally in excess of 0.35 percent after the oxygen blow.

Desulfurizing is accomplished by bringing the melt into contact with a lime containing basic slag, hereinafter referred to as a desulfurizing slag. The melt and slag are mixed and sulfur in the melt combines with calcium in the slag to form calcium sulphide, which is removed with the slag. A desulfurizing slag should be more basic than a melt-down slag. Therefore, the presence of a melt-down slag generally requires the addition of lime or other alteration of its makeup or a replacement thereof. An exemplary desulfurizing slag comprises; 50% CaO, 33% SiO 11.2% MgO, 2.7% A1 0 1% Cr O 1% FeO, 1% MnO and 0.1% P 0 In general, desulfurizing slags should have a Ca0 MgO MnO/SiO A1 0 P 0 basicity ratio of at least 1.0. A basicity ratio of at least 1.5 is, however, preferred. As stated earlier, it is advantageous to desulfurize prior to decarburizing as desulfurizing is more effective when the oxygen level of the melt and slag are low and since lower oxygen contents are present when the carbon and silicon contents are higher, as they are prior to decarburizing. In addition, early desulfurizing; i.e., desulfurizing prior to decarburizing, permits the use of carbides; e.g., calcium carbide and/or silicon carbide, as there is little worry about the consequences of carbon pickup early in the process. Carbides are advantageous in the slag since they act to return reducible oxides such as those of manganese, chromium, vanadium, tungsten, iron, etc., from the slag to the metal. Furthermore, early desulfurizing eliminates the need for a slag later in the process and shortens the period of time during which slag is present. Slags adversely affect and destroy refractory linings. Desulfurizing can be performed in the melt-down furnace or in another vessel; e.g., a ladle.

Decarburizing involves the injection of oxygen or oxygen containing material, generally gas, into the melt which is at a subatmospheric pressure, thereby precipitating a reaction between carbon within the melt and oxygen. The melt should be at a temperature of at least 2,850 F at the beginning of decarburizing. The pressure in the decarburizing vessel generally drops below 300 mm Hg. Pressure is, however, dependent upon the desired final carbon content. Lower pressures reduce the partial pressure of carbon monoxide and shift the attainable end point carbon to a lower level without necessitating oxidation of metallic components. For best results it is desirable to stir the melt. Illustrative stirring methods are inducation coil stirring, stirring caused by an inert gas stream which is injected through tuyeres or porous bricks and stirring caused by a deeply submerged lance or oxygen outlet tuyeres embedded near the bottom the decarburizing vessel. The temperature at the end of decarburizing will generally, but not necessarily, be between 2,950 and 3,150 F. Decarburizing can be performed in the melt-down furnace; e. g., an electric arc furnace fitted with two tops (a power top and a vacuum top) or in another vessel; e.g., a ladle or BOF shaped vessel fitted with a vacuum cover.

The decarburized melt is subsequently deoxidized and cast into ingots. Deoxidizing could be performed under a vacuum by adding metallic deoxidizers such as silicon and aluminum. Other known deoxidizing methods, e.g., gaseous deoxidizing, are however, also included within the scope of this invention as are any of the known casting methods.

The method of this invention can also encompass the steps of analyzing the melt and making final additions, prior to or during decarburizing, for chromium and other elements which might be desired in the steel being produced, with the exception of the highly reactive elements. As stated earlier, it is advantageous to make the additions prior to or during decarburizing as the additions can be made in the form of high carbon ferro-alloys, as there is little worry about the consequences of carbon pickup at those stages in the process. High carbon ferro-alloys are considerably less expensive than low carbon ferro-alloys. For efficiency of operation it is often desirable to desulfurize while the melt is being analyzed for final additions. At times the melt temperature has to be adjusted to accomodate the additions.

A sample of the melt can be analyzed after decarburizing in order to check the chemistry of the heat. Additions including the reactive, volatile and gaseous elements; e.g., silicon, titanium, aluminum, manganese, and nitrogen, are then added if necessary, to bring the melt to the required final analysis.

A slag reduction treatment can be interposed into the process afterthe charge is melted and before vacuum decarburization, should the slag contain an excessive amount of metallic .values. Slag reduction is accomplished by adding reducing material; e.g., ferrosilicon and/or ferrochromium silicon, to effect the return of a portion of the metallic components from the slag to the melt. No slag reduction treatment is, however, contemplated subsequent to decarburization.

The following paragraph is illustrative of a procedure encompassed by this invention.

To produce a ton heat of stainless steel consisting essentially of 18.10 18.60 percent chromium, 9.25 9.75 percent nickel, 0.07 percent maximum carbon, 0.80 1.20 percent manganese, 0.03 percent maximum phosphorus, 0.025 percent maximum sulfur, 0.30 0.60 percent silicon, balance iron, the following steps could be taken: (1) charge an electric arc furnace with 80,000 pounds (net weight) of 18 8 stainless steel scrap, 7,000 pounds (net weight) of Cr-Ni grinder dust, and 2,000 pounds (gross weight) lime; (2) .turn power on and melt charge; (3) turn power off; (4) charge 13,600 pounds (net weight) high carbon chrome, 3,400 pounds (net weight) nickel sinter, 26,480 pounds (net weight) silicon scrap and 18,000 pounds (net weight) 18 8 stainless steel scrap; (5) turn power on and melt additions; (6) turn power off; (7) add 400 pounds (gross weight) fine silicon; (8) turn power on and melt additions; (9) turn power off; 10) analyze the melt chemistry; (1 1) add lime to slag and desulfurize melt; (12) turn power on; (13) add 700 pounds (net weight) electrolytic nickel plate; (14) deslag; (15) tap melt into ladle; 16) move ladle into vacuum chamber; (17) decarburize; (18)'deoxidize; (19) analyze the melt; (20) add 1,428 pounds (net weight) of elemental manganese and 1,904 pounds (net weight) 50 percent silicon; and (21) teem into ingots.

It will be apparent to'those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention described herein.

We claim:

1. An improved method for producing stainless steel low in sulfur and carbon, which comprises the steps of: charging a furnace with materials comprised of iron, chromium, carbon and sulfur; melting said charge, said melting being partially with electric power and partially with an oxygen containing gas; desulfurizing said melt to a sulfur level below 0.04 percent; decarburizing said desulfurized melt at a temperature of at least 2,850 F in a subatmospheric pressure below 3..mm Hg with an oxygen containing gas to a carbon level below 0.15 percent; analyzing the composition of said melt and making final chromium additions prior to or during decarburizing; deoxidizing said melt; and casting said melt.

2. A method according to claim 1 wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag.

3. A method according to claim 1 wherein said melt is desulfurized to a sulfur level below 0.03 percent and wherein said desulfurizing comprises the step of mixing said melt with a lime containing slag having a Ca0 MgO MnO/SiO A1 0 P 0 basicity ratio of at least 1.0. v

4. A method according to claim 3 wherein said basicity ratio is at least 1.5.

5. A method according to claim 1 wherein said charge contains slag forming ingredients which go into the make-up of a melt-down slag, wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag and including the step of adding lime to said melt-down slag to increase its basicity.

6. A method according to claim 1 wherein said charge contains slag forming ingredients which go into i the makeup of a melt-down slag, wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag and including the step of replacing said melt-down slag with a lime containing basic slag.

7. A method according to claim 1 wherein said final chromium additions are made prior to decarburizing.

8. A method according to claim 7 wherein said melt is simultaneously analyzed and desulfurized.

9. A method according to claim 7 wherein said charge contains at least one metal from the group consisting of molybdenum, cobalt, copper, nickel and tungsten and including the step of making final additions prior to decarburizing for eachmetal of said group present in said charge.

10. A method according to claim 9 including the step of adjusting said melt temperature to accomodate said final additions made prior to decarburizing.

11. A method according to claim 1 wherein said final chromium additions are made during decarburizing.

12. A method according to claim 11 wherein said melt is simultaneously analyzed and desulfurized.

13. A method according to claim 11 wherein said charge contains at least one metal from the group consisting of molybdenum, cobalt, copper, nickel and tungsten and including the step of making final additions during decarburizing for each metal of said group present in said charge.

14. A method according to claim 13 including the step of adjusting said melt temperature to accomodate said final additions made during decarburizing.

15. A method according to claim 1 wherein said melt is desulfurized to a sulfur level below 0.03 percent prior to decarburizing, wherein said desulfurized melt is decarburized to a carbon level below 0.07 percent, and wherein said decarburizing comprises the step of injecting an oxygen containing gas into a stirred melt.

16. A method according to claim 15 wherein said melt is stirred by an induction coil and wherein the temperature at the end of decarburizing is between 2,950 and 3,150 F.

17. A method according to claim 1 wherein said charge contains at least percent of the weight of the metal to be cast.

18. A method according to claim 1 wherein said melt has a carbon content in excess of 0.35 percent immediately after melting.

19. A method according to claim 17 wherein said deoxidizing comprises the step of deoxidizing said melt in a vacuum.

20. A method according to claim 17 including the steps of analyzing said melt after decarburizing and adding at least one element from the group consisting of silicon, titanium, aluminum, manganese and nitrogen. 

2. A method according to claim 1 wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag.
 3. A method according to claim 1 wherein said melt is desulfurized to a sulfur level below 0.03 percent and wherein said desulfurizing comprises the step oF mixing said melt with a lime containing slag having a CaO + MgO + MnO/SiO2 + Al2O3 + P2O5 basicity ratio of at least 1.0.
 4. A method according to claim 3 wherein said basicity ratio is at least 1.5.
 5. A method according to claim 1 wherein said charge contains slag forming ingredients which go into the make-up of a melt-down slag, wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag and including the step of adding lime to said melt-down slag to increase its basicity.
 6. A method according to claim 1 wherein said charge contains slag forming ingredients which go into the makeup of a melt-down slag, wherein said desulfurizing comprises the step of mixing said melt with a lime containing basic slag and including the step of replacing said melt-down slag with a lime containing basic slag.
 7. A method according to claim 1 wherein said final chromium additions are made prior to decarburizing.
 8. A method according to claim 7 wherein said melt is simultaneously analyzed and desulfurized.
 9. A method according to claim 7 wherein said charge contains at least one metal from the group consisting of molybdenum, cobalt, copper, nickel and tungsten and including the step of making final additions prior to decarburizing for each metal of said group present in said charge.
 10. A method according to claim 9 including the step of adjusting said melt temperature to accomodate said final additions made prior to decarburizing.
 11. A method according to claim 1 wherein said final chromium additions are made during decarburizing.
 12. A method according to claim 11 wherein said melt is simultaneously analyzed and desulfurized.
 13. A method according to claim 11 wherein said charge contains at least one metal from the group consisting of molybdenum, cobalt, copper, nickel and tungsten and including the step of making final additions during decarburizing for each metal of said group present in said charge.
 14. A method according to claim 13 including the step of adjusting said melt temperature to accomodate said final additions made during decarburizing.
 15. A method according to claim 1 wherein said melt is desulfurized to a sulfur level below 0.03 percent prior to decarburizing, wherein said desulfurized melt is decarburized to a carbon level below 0.07 percent, and wherein said decarburizing comprises the step of injecting an oxygen containing gas into a stirred melt.
 16. A method according to claim 15 wherein said melt is stirred by an induction coil and wherein the temperature at the end of decarburizing is between 2,950* and 3,150* F.
 17. A method according to claim 1 wherein said charge contains at least 90 percent of the weight of the metal to be cast.
 18. A method according to claim 1 wherein said melt has a carbon content in excess of 0.35 percent immediately after melting.
 19. A method according to claim 17 wherein said deoxidizing comprises the step of deoxidizing said melt in a vacuum.
 20. A method according to claim 17 including the steps of analyzing said melt after decarburizing and adding at least one element from the group consisting of silicon, titanium, aluminum, manganese and nitrogen. 